With the development of long-distance optic communications technology using LD (semiconductor lasers), optical isolators have been developed in order to prevent LD noise from reflected, returning light. With the proliferation of high density communications systems, the importance of these devices has grown considerably. The basic structure consists of a Faraday rotator composed of two polarizers and a garnet crystal, along with a permanent magnet to produce the Faraday effect by the magnetization of the Faraday rotator. For the polarizer, one could use a Rochon polarizing prism, polarized beam splitter, Grant-Thompson+prism, or polarized glass depending upon application. In order to achieve small size and a high magnetic field, a rare-earth permanent magnet is used. The Faraday rotator was the part which determined the characteristics of the optical isolator. There are currently two types of materials being used. One is where the FZ method is used to produce a bulk YIG (1/2Fe.sub.5 O.sub.12) single crystal and the other is where the liquid phase epitaxial (LPE) method is used in order to produce a BiRIG (rare earth bismuth-ison garnet) film on a garnet type of substrate. The Faraday rotation angle .theta..sub.f is proportional to the thickness of the crystals, and .theta..sub.f per unit of length is different from each material. In order to obtain an angle .theta..sub.f =45.degree. as required for an optical isolator, the YIG should be about 2 mm, and the BiRIG should be 200 to 500 .mu.m. In consideration of mass production and lowered costs, after the FZ method has been used to obtain the bulk YIG, in order to produce the required shape on the substrate using the LPE method without wasteful machining, it is possible to produce a large film 1/4 of the thickness but equivalent to the YIG in the Faraday rotator. This is of greet benefit for the economic proliferation of these elements. However, in u these YIG and LPE garnet crystals, there have some differences in optical characteristics which result in temperature or wavelength dependence in the Faraday rotation. FIG. shows the temperature dependence (a) and the wavelength dependence (b) of .theta..sub.f for a BiRIG. Depending on the materials, there are variations in the reverse slope, but generally, the .theta..sub.f corresponds to temperature and wavelength. Optical isolators are adjusted and assembled to have a maximum isolation at the wavelength and the ambient temperature which they are assembled. However, in temperature ranges from 0.degree. to 70.degree. C. where these devices will be practically used, near the temperatures at either end of this range (0.degree. C. and 70.degree. C.) the isolation characteristics tend to deteriorate. In YIG crystals, the Faraday rotation temperature dependence coefficient is generally K.sub.T =-0.04 deg/.degree. C. With the LPE method, in materials where there is essentially little absorption, it is K.sub.T =-0.04 to -0.07 deg/.degree. C. When the optical isolators are assembled at room temperature (about 23.degree. C.), they are adjusted so that .theta..sub.f =45.degree. at 23.degree. C. However, if we assume the temperature coefficient of an LPE garnet element to be -0.07 deg/.degree. C., then at the upper end of 23.degree. C. .+-.20.degree. C. (eg. 43.degree. C.), .theta..sub.f =45 -0.07.times.20=43.6.degree.. At the lower limit of 3.degree. C., this becomes 46.4.degree.. This greatly degrades the isolation due to the slippage from .theta..sub.f =45.degree.. In principle, isolation is -10 Log[sin.sup.2 (45-.theta..sub.f)], so in the previous example, at 3.degree. C. and 43.degree. C. it would be 32 dB. The graph in FIG. 2 (1) shows the temperature dependence of isolation for a typical optical isolator. The peak of isolation is at 23.degree. C., and it falls off below and above that temperature in a nearly symmetrical curve. In this case, when considering a temperature range from 0.degree. to 76.degree. C., at the limit temperatures, the Faraday rotation angle is 44 deg for 24.fwdarw.0.degree. C. and 46 deg from 24.fwdarw.70.degree. C. Table 1 shows the isolation at 0.degree. C. and 70.degree. C. when the isolation has an angular displacement angle .DELTA..theta. from 45.degree. according to -10 log (sin.sup.2 .DELTA..theta.) for YIG and LPE garnets (when the temperature coefficient=-0.06 deg/.degree.C.).
TABLE 1 ______________________________________ Temp. Coefficient Isolation (dB) (deg/.degree.C.) 0.degree. C. 70.degree. C. ______________________________________ YIG garnet -0.04 -35.5 -29.9 LPE garnet -0.06 -32.0 -26.3 ______________________________________
If we assume that 30 dB or greater of isolation is required from O.fwdarw.70.degree. C., then problems would appear with either method at the high temperature end of the range. In order to reduce this problem, the materials used in the Faraday rotator should have a low temperature coefficient, but at the current time, such low absorption materials are not available. The following methods can be considered as ways of providing high isolation at a temperature range of 0.degree. to 70.degree. C.:
1) Assemble the optical isolators at a temperature mid-way in the above range, eg. 35.degree. C.
2) Produce isolators with a maximum isolation temperature of 35.degree. C. by moving the polarizer angle of rotation .DELTA..theta. from an angle of 45.degree..
3) Produce isolators with a maximum isolation temperature of 35.degree. C. by deviating .DELTA..theta. from the 45.degree. Faraday rotation angle.
If the first of the above methods were used, an assembly system would have to be established where the ambient temperature was higher than room temperature. If the method 2) were adopted, during the mechanical displacement by .DELTA..theta. from 45.degree., it would be impossible to fix the peak for the standard isolation, so the resulting products would have some fluctuation in their characteristics. In the case of 3), it would involve increasing or reducing the thickness of the Faraday rotator, and the .theta..+-..DELTA..theta. adjustment would be complex. In any of the above cases, it would not be practical to implement such production.
Considering costs and production for a multi-purpose optical isolator (for optic communications of subscribers, Cable TV optic communication systems), it would be difficult to meet the requirements except by using the LPE method garnet. Thus, there is a demand for optical isolators which are produced using the LPE method but which have little absorption and are stable in the face of temperature variations.